Method and device for determining safe threshold for power grid voltage of wind farm converging area

ABSTRACT

Disclosed are a method and a device for determining safe threshold for power grid voltage of a wind farm converging area, The method includes: obtaining ground state voltage; obtaining upper and lower limit voltage of the output bus, first lower and upper limit of reactive powers of fans and compensation devices, and lower and upper limit of active power fluctuations; obtaining second lower and upper limit of reactive powers of fans and compensation devices; determining whether voltage error is greater than a predetermined threshold; replacing the first lower and upper limit of the reactive powers of fans and the reactive powers of compensation devices with the second lower and upper limit of reactive powers of fans and compensation devices respectively, and repeating above steps if the voltage error is greater than the predetermined threshold, else defining the upper and lower limit voltage of the output bus as a safe threshold.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and benefits of Chinese PatentApplication No. 201410426310.X, filed with the State IntellectualProperty Office of P. R. China on Aug. 26, 2014, the entire contents ofwhich are incorporated herein by reference.

FIELD

The present disclosure relates to a field of a security and control of apower system, and more particularly relates to a method for determininga safe threshold for power grid voltage of a wind farm converging areaand a device for determining a safe threshold for power grid voltage ofa wind farm converging area.

BACKGROUND

In recent years, cascading trip-off of wind farms frequently occurs,thus bringing a challenge to security of wind farms, so that a voltagecontrol of wind farms is particularly important. If the use of windenergy is mainly realized by sending wind energy into power grids on alarge scale, since wind farms are located at end of a weak gridstructure and wind power fluctuates widely, when a wind farm is trippedoff, the other wind farms would be affected. However, a main network andwind farms are controlled by two different automatic voltage controlsystems and coordination control of the two systems is lacking, thussafe thresholds for voltages of the main network and wind farms areadjusted according to experience.

SUMMARY

According to embodiments of a first aspect of the present disclosure,there is provided a method for determining a safe threshold for powergrid voltage of a wind farm converging area including: step A: obtaininga ground state voltage of an output bus of each wind farm in the windfarm converging area; step B: obtaining an upper limit voltage and alower limit voltage of the output bus of each wind farm, a first lowerlimit and a first upper limit of reactive powers of fans in each windfarm, a first lower limit and a first upper limit of reactive powers ofcompensation devices in each wind farm, and a lower limit and an upperlimit of active power fluctuations of the fans in each wind farmaccording to the ground state voltage of the output bus of each windfarm; step C: obtaining a second lower limit and a second upper limit ofthe reactive powers of the fans in each wind farm, a second lower limitand a second upper limit of the reactive powers of the compensationdevices in each wind farm according to the upper limit voltage and thelower limit voltage of the output bus of each wind farm, the first lowerlimit and the first upper limit of the reactive powers of the fans ineach wind farm, the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm, and thelower limit and the upper limit of the active power fluctuations of thefans in each wind farm; step D: obtaining a voltage error according tothe upper limit voltage and the lower limit voltage of the output bus ofeach wind farm; step E: determining whether the voltage error is greaterthan a predetermined threshold; step F: replacing the first lower limitand the first upper limit of the reactive powers of the fans in eachwind farm, and the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm with thesecond lower limit and the second upper limit of the reactive powers ofthe fans in each wind farm, the second lower limit and the second upperlimit of the reactive powers of the compensation devices in each windfarm respectively, and repeating steps B-F to perform a time iteration,if the voltage error is greater than the predetermined threshold; stepG: defining the upper limit voltage and the lower limit voltage of theoutput bus of each wind farm as a safe threshold of each wind farm, ifthe voltage error is less than or equal to the predetermined threshold.

According to embodiments of a second aspect of the present disclosure,there is provided a device for determining a safe threshold for powergrid voltage of a wind farm converging area, including: a non-transitorycomputer-readable medium comprising computer-executable instructionsstored thereon; and an instruction execution system, which is configuredby the instructions to implement at least one of following modules: afirst obtaining module, configured to obtain a ground state voltage ofan output bus of each wind farm in the wind farm converging area; asecond obtaining module, configured to obtain an upper limit voltage anda lower limit voltage of the output bus of each wind farm, a first lowerlimit and a first upper limit of reactive powers of fans in each windfarm, a first lower limit and a first upper limit of reactive powers ofcompensation devices in each wind farm, and a lower limit and an upperlimit of active power fluctuations of the fans in each wind farmaccording to the ground state voltage of the output bus of each windfarm; a third obtaining module, configured to obtain a second lowerlimit and a second upper limit of the reactive powers of the fans ineach wind farm, a second lower limit and a second upper limit of thereactive powers of the compensation devices in each wind farm accordingto the upper limit voltage and the lower limit voltage of the output busof each wind farm, the first lower limit and the first upper limit ofthe reactive powers of the fans in each wind farm, the first lower limitand the first upper limit of the reactive powers of the compensationdevices in each wind farm, and the lower limit and the upper limit ofthe active power fluctuations of the fans in each wind farm; a fourthobtaining module, configured to obtain a voltage error according to theupper limit voltage and the lower limit voltage of the output bus ofeach wind farm; a determining module, configured to determine whetherthe voltage error is greater than a predetermined threshold; aniteration module, configured to replace the first lower limit and thefirst upper limit of the reactive powers of the fans in each wind farm,and the first lower limit and the first upper limit of the reactivepowers of the compensation devices in each wind farm with the secondlower limit and the second upper limit of the reactive powers of thefans in each wind farm, the second lower limit and the second upperlimit of the reactive powers of the compensation devices in each windfarm respectively, and to repeat to perform a time iteration, if thevoltage error is greater than the predetermined threshold; a definingmodule, configured to define the upper limit voltage and the lower limitvoltage of the output bus of each wind farm as a safe threshold of eachwind farm, if the voltage error is less than or equal to thepredetermined threshold.

According to embodiments of a third aspect of the present disclosure,there is provided a non-transitory computer-readable storage mediumhaving stored therein instructions that, when executed by a processor ofa computer, causes the computer to perform a method for determining asafe threshold for power grid voltage of a wind farm converging area,the method including: step A: obtaining a ground state voltage of anoutput bus of each wind farm in the wind farm converging area; step B:obtaining an upper limit voltage and a lower limit voltage of the outputbus of each wind farm, a first lower limit and a first upper limit ofreactive powers of fans in each wind farm, a first lower limit and afirst upper limit of reactive powers of compensation devices in eachwind farm, and a lower limit and an upper limit of active powerfluctuations of the fans in each wind farm according to the ground statevoltage of the output bus of each wind farm; step C: obtaining a secondlower limit and a second upper limit of the reactive powers of the fansin each wind farm, a second lower limit and a second upper limit of thereactive powers of the compensation devices in each wind farm accordingto the upper limit voltage and the lower limit voltage of the output busof each wind farm, the first lower limit and the first upper limit ofthe reactive powers of the fans in each wind farm, the first lower limitand the first upper limit of the reactive powers of the compensationdevices in each wind farm, and the lower limit and the upper limit ofthe active power fluctuations of the fans in each wind farm; step D:obtaining a voltage error according to the upper limit voltage and thelower limit voltage of the output bus of each wind farm; step E:determining whether the voltage error is greater than a predeterminedthreshold; step F: replacing the first lower limit and the first upperlimit of the reactive powers of the fans in each wind farm, and thefirst lower limit and the first upper limit of the reactive powers ofthe compensation devices in each wind farm with the second lower limitand the second upper limit of the reactive powers of the fans in eachwind farm, the second lower limit and the second upper limit of thereactive powers of the compensation devices in each wind farmrespectively, and repeating steps B-F to perform a time iteration, ifthe voltage error is greater than the predetermined threshold; step G:defining the upper limit voltage and the lower limit voltage of theoutput bus of each wind farm as a safe threshold of each wind farm, ifthe voltage error is less than or equal to the predetermined threshold.

The technical solutions provided by embodiments of the presentdisclosure have following advantageous effects.

(1) By both taking voltage security constraints of wind farms and a mainnetwork into account, when a wind farm is tripped off, the other windfarms continue to work normally. A safe threshold for power grid voltagemay be calculated by a rapid iteration. The calculated safe threshold isrobust to active powers of the wind farms, thus being beneficial tocontrol the power grid voltage.

(2) Information exchange between the wind farms and a station of thewind farm converging area may be implemented by these technicalsolutions. Calculations in these technical solutions are small andrapid, which is beneficial to control the power grid voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages of embodiments of the presentdisclosure will become apparent and more readily appreciated from thefollowing descriptions made with reference to the accompanying drawings,in which:

FIG. 1 is a flow chart of the method for determining a safe thresholdfor power grid voltage of a wind farm converging area according toembodiments of the present disclosure.

FIG. 2 is a block diagram of the device for determining a safe thresholdfor power grid voltage of a wind farm converging area according toembodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will be made in detail to embodiments of the presentdisclosure, where the same or similar elements and the elements havingsame or similar functions are denoted by like reference numeralsthroughout the descriptions. The embodiments described herein withreference to drawings are explanatory, illustrative, and used togenerally understand the present disclosure. The embodiments shall notbe construed to limit the present disclosure.

In the description of the present disclosure, it should be understoodthat, terms such as “first” and “second” are used herein for purposes ofdescription, and are not intended to represent or indicate relativeimportance or significance or to represent or indicate numbers orlocations. In the description of the present disclosure, it should beunderstood that, unless specified or limited otherwise, terms such as“connected” and “coupled” should be understood broadly, and may be, forexample, fixed connections, detachable connections, or integralconnections; or may be mechanical or electrical connections; or may bedirect connections or indirect connections via intervening structures,which can be understood by those skilled in the art according tospecific situations. Moreover, in the description of the presentinvention, unless specified otherwise, “a plurality of” means two ormore than two.

Any process or method described in a flow chart or described herein inother ways may be understood to include one or more modules, segments orportions of codes of executable instructions for achieving specificlogical functions or steps in the process. Although the flow chart showsa specific order of execution, it is understood that the order ofexecution may differ from what is depicted. For example, the order ofexecution of two or more boxes may be scrambled relative to the ordershown.

In the following, a method for determining a safe threshold for powergrid voltage of a wind farm converging area according to embodiments ofthe present disclosure will be described in detail with reference todrawings.

As shown in FIG. 1, FIG. 1 is a flow chart of the method for determininga safe threshold for power grid voltage of a wind farm converging areaaccording to embodiments of the present disclosure. The method fordetermining a safe threshold for power grid voltage of a wind farmconverging area includes following steps.

At step S101: a ground state voltage of an output bus of each wind farmin the wind farm converging area is obtained.

In an embodiment, step S101 may include following steps.

1. Each reactive power of each fan in each wind farm is set to be 0, andeach reactive power of each compensation device in each wind farm is setto be 0.

2. The ground state voltage of the output bus of each wind farm isobtained by performing a ground state flow power calculation.

At step S102: an upper limit voltage and a lower limit voltage of theoutput bus of each wind farm, a first lower limit and a first upperlimit of reactive powers of fans in each wind farm, a first lower limitand a first upper limit of reactive powers of compensation devices ineach wind farm, and a lower limit and an upper limit of active powerfluctuations of the fans in each wind farm are obtained according to theground state voltage of the output bus of each wind farm.

In an embodiment, step S102 may include following steps.

1. In order to ensure the fans in each wind farm can not be tripped offthe power grid in a normal operation, a first objective function (1) ofthe upper limit voltage of the output bus of each wind farm and a secondobjective function (2) of the lower limit voltage of the output bus ofeach wind farm are established:

$\begin{matrix}{\overset{\_}{V_{{PCC},i}} = {{\min\limits_{\Delta \; p_{w_{i}}}{\min\limits_{{\Delta \; q_{w_{i}}},{\Delta q}_{\; c_{i}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{j \in c_{i}}{\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (1) \\{\underset{\_}{V_{{PCC},i}} = {{\min\limits_{\Delta \; p_{w_{i}}}{\min\limits_{{\Delta \; q_{w_{i}}},{\Delta q}_{\; c_{i}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}{\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (2)\end{matrix}$

where V_(PCC,i) is an upper limit voltage of the output bus of a i^(th)wind farm, V_(PCC,i) is a lower limit voltage of the output bus of thei^(th) wind farm, V_(PCC,i) ⁰ is a ground state of the i^(th) wind farm,w_(i) is a set of fans in the i^(th) wind farm, c_(i) is a set ofcompensation devices in the i^(th) wind farm,

$\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}$

represent a voltage sensitivity coefficient of an active power of aj^(th) fan in the i^(th) wind farm relative to V_(PCC,i) ⁰ and a voltagesensitivity coefficient of a reactive power of the j^(th) fan in thei^(th) wind farm relative to V_(PCC,i) ⁰ respectively,

$\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}$

represents a voltage sensitivity coefficient of a reactive power of ak^(th) compensation device in the i^(th) wind farm relative to V_(PCC,i)⁰, Δp_(i,j) and Δq_(i,j) represent a variation of the active power ofthe j^(th) fan in the i^(th) wind farm and a variation of the reactivepower of the j^(th) fan in the i^(th) wind farm respectively, Δq_(i,k)represents a variation of the reactive power of the k^(th) compensationdevice in the i^(th) wind farm.

2. first boundary conditions (3) for the first objective function andthe second objective function is determined:

$\begin{matrix}{{{s.t.\mspace{14mu} \underset{\_}{V_{i,t}^{0}}} \leq {V_{i,t}^{0} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + \mspace{14mu} {\underset{k \in c_{i}}{\quad\sum}\; \frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}} \leq \overset{\_}{V_{i,t}^{0}}},{{t \in w_{i}}{{\underset{\_}{\Delta \; p_{i,t}} \leq {\Delta \; p_{i,t}} \leq \overset{\_}{\Delta \; p_{i,t}}},\; {t \in w_{i}}}{{\underset{\_}{\Delta \; q_{i,t}} \leq {\Delta \; q_{i,t}} \leq \overset{\_}{\Delta \; q_{i,t}}},\; {t \in w_{i}}}{{\underset{\_}{\Delta \; q_{i,k}} \leq {\Delta \; q_{i,k}} \leq \overset{\_}{\Delta \; q_{i,k}}},\; {k \in c_{i}}}{\underset{\_}{\Delta \; Q_{wi}} \leq {\sum\limits_{t \in w_{i}}\; {\Delta \; q_{i,t}}} \leq \overset{\_}{\Delta \; Q_{wi}}}{\underset{\_}{\Delta \; Q_{ci}} \leq {\sum\limits_{k \in c_{i}}\; {\Delta \; q_{i,k}}} \leq \overset{\_}{\Delta \; Q_{ci}}}}} & (3)\end{matrix}$

where V_(i,t) ⁰ and V_(i,t) ⁰ represent a lower limit voltage and upperlimit voltage in a normal operation of a t^(th) fan in the i^(th) windfarm respectively, t≠j,

$\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}$

represent a voltage sensitivity coefficient of the active power of thej^(th) fan in the i^(th) wind farm relative to the t^(th) fan and avoltage sensitivity coefficient of the reactive power of the j^(th) fanin the i^(th) wind farm relative to the t^(th) fan respectively,

$\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}$

represents a voltage sensitivity coefficient of the reactive power ofthe k^(th) compensation device in the i^(th) wind farm relative to thet^(th) fan, Δp_(i,t) and Δp_(i,t) represent a lower limit and an upperlimit of an active power fluctuation of the t^(th) fan in the i^(th)wind farm respectively, Δq_(i,t) and Δq_(i,t) represent a lower limitand an upper limit of a reactive power fluctuation of the t^(th) fan inthe i^(th) wind farm respectively, Δq_(i,k) and Δq_(i,k) represent alower limit and an upper limit of a reactive power fluctuation of thek^(th) compensation device in the i^(th) wind farm respectively, ΔQ_(wi)and ΔQ_(wi) represent a lower limit and an upper limit of reactivepowers of fans in the i^(th) wind farm, ΔQ_(ci) and ΔQ_(ci) represent alower limit and an upper limit of reactive powers of compensationdevices in the i^(th) wind farm;

3. a lower limit of the active power fluctuation of the i^(th) wind farmis calculated according to formula (4):

$\begin{matrix}{{\underset{\_}{\Delta \; P_{i}} = {\sum\limits_{t \in w_{i}}\; \underset{\_}{\Delta \; p_{i,t}}}},} & (4)\end{matrix}$

and an upper limit of the active power fluctuation of the i^(th) windfarm according to formula (5):

$\begin{matrix}{{\overset{\_}{\Delta \; P_{i}} = {\sum\limits_{t \in w_{i}}\; \overset{\_}{\Delta \; p_{i,t}}}},} & (5)\end{matrix}$

where ΔP_(i) and ΔP_(i) represent the lower limit and the upper limit ofthe fluctuation of the active power of the i^(th) wind farmrespectively.

4. the first objective function and the second objective function isresolved based on the first boundary conditions to obtain obtain theupper limit voltage and the lower limit voltage of the output bus ofeach wind farm, the first lower limit and the first upper limit of thereactive powers of the fans in each wind farm, the first lower limit andthe first upper limit of the reactive powers of the compensation devicesin each wind farm, and the lower limit and the upper limit of the activepower fluctuations of the fans in each wind farm.

At step S103: a second lower limit and a second upper limit of thereactive powers of the fans in each wind farm, a second lower limit anda second upper limit of the reactive powers of the compensation devicesin each wind farm are obtained according to the upper limit voltage andthe lower limit voltage of the output bus of each wind farm, the firstlower limit and the first upper limit of the reactive powers of the fansin each wind farm, the first lower limit and the first upper limit ofthe reactive powers of the compensation devices in each wind farm, andthe lower limit and the upper limit of the active power fluctuations ofthe fans in each wind farm.

In an embodiment, step S103 may include following steps.

1. A third objective function (6) of reactive power in the wind farmconverging area is established according to the upper limit voltage andthe lower limit voltage of the output bus of each wind farm, the firstlower limit and the first upper limit of the reactive powers of the fansin each wind farm, the first lower limit and the first upper limit ofthe reactive powers of the compensation devices in each wind farm, andthe lower limit and the upper limit of the active power fluctuation ofthe fans in each wind farm:

$\begin{matrix}{{\Delta \; Q} = {{\max\limits_{\Delta \; P}\mspace{11mu} {\min\limits_{{\Delta \; Q_{wi}},{\Delta \; Q_{ci}}}{\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{wi}^{\max}} - {\Delta \; Q_{wi}^{\min}}} \right)}}} + {\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{ci}^{\max}} - {\Delta \; Q_{ci}^{\min}}} \right)}}} & (6)\end{matrix}$

where ΔQ_(wi) ^(max) and ΔQ_(wi) ^(min) represent a second lower limitand a second upper limit of reactive powers of fans in the i^(th) windfarm, ΔQ_(ci) ^(max) and ΔQ_(ci) ^(min) represent a second lower limitand a second upper limit of reactive powers of compensation devices inthe i^(th) wind farm.

2. Second boundary conditions (7) for the third objective function aredetermined:

$\begin{matrix}{{{{s.t.\mspace{14mu} \underset{\_}{V_{{PCC},i}}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{n \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{i,n}}\Delta \; Q_{n}}} \right)}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w}}{{\underset{\_}{V_{{PCC},i}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{{n \in w},{n \neq s}}\left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}\Delta \; Q_{n}}} \right)} + {\frac{\partial V_{{PCC},i}^{0}}{\partial P_{s}}\Delta \; P_{s}^{s}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{s}}\Delta \; Q_{s}^{s}}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w},{i \neq s},{\in S}}{{\underset{\_}{\Delta \; P_{i}} \leq \; {\Delta \; P_{i}} \leq \overset{\_}{\Delta \; P_{i}}},{i \in w}}{{\underset{\_}{\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\min}} \leq {\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\max}} \leq \overset{\_}{\Delta \; Q_{wi}}},{i \in w}}{{\underset{\_}{\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\min}} \leq {\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\max}} \leq \overset{\_}{\Delta \; Q_{ci}}},{i \in w}}} & (7)\end{matrix}$

where

$\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}$

represent a voltage sensitivity coefficient of an active power of an^(th) wind farm relative to the output bus of the i^(th) wind farm anda voltage sensitivity coefficient of a reactive power of the n^(th) windfarm relative to the output bus of the i^(th) wind farm respectively,ΔP_(n) and ΔQ_(n) represent a variation of the active power of then^(th) wind farm and a variation of the reactive power of the n^(th)wind farm respectively, n≠i, w is a set of wind farms in the wind farmconverging area, S is a set of tripped off wind farms, ΔP_(s) ^(s) andΔQ_(s) ^(s) represent a variation of active powers of tripped off windfarms and a variation of reactive powers of tripped off wind farmsrespectively.

3. The third objective function is resolved based on the second boundaryconditions to obtain the second lower limit and the second upper limitof the reactive powers of the fans in each wind farm, the second lowerlimit and the second upper limit of the reactive powers of thecompensation devices in each wind farm.

At step S104, a voltage error is obtained according to the upper limitvoltage and the lower limit voltage of the output bus of each wind farm.

The voltage error may be obtained according to formula (8):

$\begin{matrix}{{\Delta V}_{S} = {{\sum\limits_{i \in w}\; {{\overset{\_}{V_{{PCC},i}^{({m + 1})}} - \overset{\_}{V_{{PCC},i}^{(m)}}}}} + {\sum\limits_{i \in w}\; {{\underset{\_}{V_{{PCC},i}^{({m + 1})}} - \underset{\_}{V_{{PCC},i}^{(m)}}}}}}} & (8)\end{matrix}$

where V_(PCC,i) ^((m)) is an upper limit voltage of the output bus ofthe i^(th) wind farm in a m^(th) time iteration and V_(PCC,i) ^((m)) isa lower limit voltage of the output bus of the i^(th) wind farm in them^(th) time iteration.

At step S105, it is determined whether the voltage error is greater thana predetermined threshold.

A predetermined threshold ε is determined, it is determined whetherΔV_(S) is greater than ε.

At step S106, the first upper limit of the reactive powers of the fansin each wind farm, and the first lower limit and the first upper limitof the reactive powers of the compensation devices in each wind farm arereplaced with second lower limit and the second upper limit of thereactive powers of the fans in each wind farm, the second lower limitand the second upper limit of the reactive powers of the compensationdevices in each wind farm respectively, and steps S102-S107 are repeatedto perform a time iteration, if the voltage error is greater than thepredetermined threshold.

If ΔV_(S) is greater than ε, the first lower limit ΔQ_(wi) and the firstupper limit ΔQ_(wi) of the reactive powers of the fans in each windfarm, and the first lower limit ΔQ_(ci) and the first upper limitΔQ_(ci) of the reactive powers of the compensation devices in each windfarm are replace with the second lower limit ΔQ_(wi) ^(min) and thesecond upper limit ΔQ_(wi) ^(max) the reactive powers of the fans ineach wind farm, the second lower limit ΔQ_(ci) ^(min) and the secondupper limit ΔQ_(ci) ^(max) of the reactive powers of the compensationdevices in each wind farm respectively, and steps S102-S107 are repeatedto perform a time iteration.

At step S107, the upper limit voltage and the lower limit voltage of theoutput bus of each wind farm are defined as a safe threshold of eachwind farm, if the voltage error is less than or equal to thepredetermined threshold if the voltage error is less than or equal tothe predetermined threshold.

The upper limit voltage of the output bus of each wind farm is definedas an upper limit of the safe threshold of each wind farm, and the lowerlimit voltage of the output bus of each wind farm is defined as a lowerlimit of the safe threshold of each wind farm.

FIG. 2 is a block diagram of the device for determining a safe thresholdfor power grid voltage of a wind farm converging area according toembodiments of the present disclosure. As shown in FIG. 2, the deviceincludes: a first obtaining module 21, a second obtaining module 22, athird obtaining module 23, a fourth obtaining module 24, a determiningmodule 25, an iteration module 26 and a defining module 27.

The first obtaining module 21 is configured to obtain a ground statevoltage of an output bus of each wind farm in the wind farm convergingarea.

The second obtaining module 22 is configured to obtain an upper limitvoltage and a lower limit voltage of the output bus of each wind farm, afirst lower limit and a first upper limit of reactive powers of fans ineach wind farm, a first lower limit and a first upper limit of reactivepowers of compensation devices in each wind farm, and a lower limit andan upper limit of active power fluctuations of the fans in each windfarm according to the ground state voltage of the output bus of eachwind farm.

The third obtaining module 23 is configured to a second lower limit anda second upper limit of the reactive powers of the fans in each windfarm, a second lower limit and a second upper limit of the reactivepowers of the compensation devices in each wind farm according to theupper limit voltage and the lower limit voltage of the output bus ofeach wind farm, the first lower limit and the first upper limit of thereactive powers of the fans in each wind farm, the first lower limit andthe first upper limit of the reactive powers of the compensation devicesin each wind farm, and the lower limit and the upper limit of the activepower fluctuations of the fans in each wind farm.

The fourth obtaining module 24 is configured to obtain a voltage erroraccording to the upper limit voltage and the lower limit voltage of theoutput bus of each wind farm.

The determining module 25 is configured to determine whether the voltageerror is greater than a predetermined threshold.

The iteration module 26 is configured to replace the first lower limitand the first upper limit of the reactive powers of the fans in eachwind farm, and the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm with thesecond lower limit and the second upper limit of the reactive powers ofthe fans in each wind farm, the second lower limit and the second upperlimit of the reactive powers of the compensation devices in each windfarm respectively, and to repeat to perform a time iteration, if thevoltage error is greater than the predetermined threshold.

The defining module 27 is configured to define the upper limit voltageand the lower limit voltage of the output bus of each wind farm as asafe threshold of each wind farm, if the voltage error is less than orequal to the predetermined threshold.

In an embodiment, the first obtaining module 21 includes: a setting unitand a first obtaining unit. The a setting unit configured to set eachreactive power of each fan in each wind farm to be 0, and to set eachreactive power of each compensation device in each wind farm to be 0; afirst obtaining unit is configured to obtain the ground state voltage ofthe output bus of each wind farm by performing a ground state flow powercalculation.

In an embodiment, the second obtaining module 22 includes: a firstestablishing unit, a first determining unit, a first calculating unitand a first resolving unit.

The first establishing unit is configured to establish a first objectivefunction (1) of the upper limit voltage of the output bus of each windfarm and a second objective function (2) of the lower limit voltage ofthe output bus of each wind farm:

$\begin{matrix}{\overset{\_}{V_{{PCC},i}} = {{\min\limits_{\Delta \; p_{w_{i}}}{\max\limits_{\Delta_{q_{w_{i}}},{\Delta \; q_{c_{i}}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{j \in c_{i}}{\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (1) \\{\underset{\_}{V_{{PCC},i}} = {{\max\limits_{\Delta \; p_{w_{i}}}{\min\limits_{\Delta_{q_{w_{i}}},{\Delta \; q_{c_{i}}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}{\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (2)\end{matrix}$

where V_(PCC,i) is an upper limit voltage of the output bus of a i^(th)wind farm, V_(PCC,i) is a lower limit voltage of the output bus of thei^(th) wind farm, V_(PCC,i) ⁰ is a ground state of the i^(th) wind farm,w_(i) is a set of fans in the i^(th) wind farm, c_(i) is a set ofcompensation devices in the i^(th) wind farm,

$\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}$

represent a voltage sensitivity coefficient of an active power of aj^(th) fan in the i^(th) wind farm relative to V_(PCC,i) ⁰ and a voltagesensitivity coefficient of a reactive power of the j^(th) fan in thei^(th) wind farm relative to V_(PCC,i) ⁰ respectively,

$\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}$

represents a voltage sensitivity coefficient of a reactive power of ak^(th) compensation device in the i^(th) wind farm relative to V_(PCC,i)⁰, Δp_(i,j) and Δq_(i,j) represent a variation of the active power ofthe j^(th) fan in the i^(th) wind farm and a variation of the reactivepower of the j^(th) fan in the i^(th) wind farm respectively, Δq_(i,k)represents a variation of the reactive power of the k^(th) compensationdevice in the i^(th) wind farm.

The first determining unit is configured to determine first boundaryconditions (3) for the first objective function and the second objectivefunction:

$\begin{matrix}{{{{s.t.\mspace{14mu} \underset{\_}{V_{i,t}^{0}}} \leq {V_{i,t}^{0} + {\sum\limits_{j \in w_{i}}\left( {{\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}{\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}} \leq \overset{\_}{V_{i,t}^{0}}},{t \in w_{i}}}{{\underset{\_}{\Delta \; p_{i,t}} \leq {\Delta \; p_{i,t}} \leq {\overset{\_}{{\Delta \; p_{i,t}},}\mspace{14mu} t}} \in w_{i}}{{\underset{\_}{\Delta \; q_{i,t}} \leq {\Delta \; q_{i,t}} \leq {\overset{\_}{{\Delta \; q_{i,t}},}\mspace{14mu} t}} \in w_{i}}{{\underset{\_}{\Delta \; q_{i,k}} \leq {\Delta \; q_{i,k}} \leq {\overset{\_}{{\Delta \; q_{i,k}},}\mspace{14mu} k}} \in c_{i}}{\underset{\_}{\Delta \; Q_{wi}} \leq {\sum\limits_{t \in w_{i}}\; {\Delta \; q_{i,t}}} \leq \overset{\_}{\Delta \; Q_{wi}}}{\underset{\_}{\Delta \; Q_{ci}} \leq {\sum\limits_{k \in c_{i}}\; {\Delta \; q_{i,k}}} \leq \overset{\_}{\Delta \; Q_{ci}}}} & (3)\end{matrix}$

where V_(i,t) ⁰ and V_(i,t) ⁰ represent a lower limit voltage and upperlimit voltage in a normal operation of a t^(th) fan in the i^(th) windfarm respectively, t≠j,

$\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}$

represent a voltage sensitivity coefficient of the active power of thej^(th) fan in the i^(th) wind farm relative to the t^(th) fan and avoltage sensitivity coefficient of the reactive power of the j^(th) fanin the i^(th) wind farm relative to the t^(th) fan respectively,

$\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}$

represents a voltage sensitivity coefficient of the reactive power ofthe k^(th) compensation device in the i^(th) wind farm relative to thet^(th) fan, Δp_(i,t) and Δp_(i,t) represent a lower limit and an upperlimit of an active power fluctuation of the t^(th) fan in the i^(th)wind farm respectively, Δq_(i,t) and Δq_(i,t) represent a lower limitand an upper limit of a reactive power fluctuation of the t^(th) fan inthe i^(th) wind farm respectively, Δq_(i,k) and Δq_(i,k) represent alower limit and an upper limit of a reactive power fluctuation of thek^(th) compensation device in the i^(th) wind farm respectively, ΔQ_(wi)and ΔQ_(wi) represent a lower limit and an upper limit of reactivepowers of fans in the i^(th) wind farm, ΔQ_(ci) and ΔQ_(ci) represent alower limit and an upper limit of reactive powers of compensationdevices in the i^(th) wind farm.

The first calculating unit is configured to

calculate a lower limit of the active power fluctuation of the i^(th)wind farm according to formula (4):

$\begin{matrix}{{\underset{\_}{\Delta \; P_{i}} = {\sum\limits_{t \in w_{i}}\; \underset{\_}{\Delta \; p_{i,t}}}},} & (4)\end{matrix}$

and to calculate an upper limit of the active power fluctuation of thei^(th) wind farm according to formula (5):

$\begin{matrix}{{\overset{\_}{\Delta \; P_{i}} = {\sum\limits_{t \in w_{i}}\; \overset{\_}{\Delta \; p_{i,t}}}},} & (5)\end{matrix}$

where ΔP_(i) and ΔP_(i) represent the lower limit and the upper limit ofthe active power fluctuation of the i^(th) wind farm respectively.

The first resolving unit is configured to resolve the first objectivefunction and the second objective function based on the first boundaryconditions to obtain the upper limit voltage and the lower limit voltageof the output bus of each wind farm, the first lower limit and the firstupper limit of the reactive powers of the fans in each wind farm, thefirst lower limit and the first upper limit of the reactive powers ofthe compensation devices in each wind farm, and the lower limit and theupper limit of the active power fluctuations of the fans in each windfarm.

In an embodiment, the third obtaining module 23 includes: a secondestablishing unit, a second determining unit and a second resolvingunit.

The second establishing unit is configured to establish a thirdobjective function (6) of reactive power in the wind farm convergingarea according to the upper limit voltage and the lower limit voltage ofthe output bus of each wind farm, the first lower limit and the firstupper limit of the reactive powers of the fans in each wind farm, thefirst lower limit and the first upper limit of the reactive powers ofthe compensation devices in each wind farm, and the lower limit and theupper limit of the active power fluctuation of the fans in each windfarm:

$\begin{matrix}{{\Delta \; Q} = {{\max\limits_{\Delta \; P}\mspace{11mu} {\min\limits_{{\Delta \; Q_{wi}},{\Delta \; Q_{ci}}}{\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{wi}^{\max}} - {\Delta \; Q_{wi}^{\min}}} \right)}}} + {\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{ci}^{\max}} - {\Delta \; Q_{ci}^{\min}}} \right)}}} & (6)\end{matrix}$

where ΔQ_(wi) ^(max) and ΔQ_(wi) ^(min) represent a second lower limitand a second upper limit of reactive powers of fans in the i^(th) windfarm, ΔQ_(ci) ^(max) and ΔQ_(ci) ^(min) represent a second lower limitand a second upper limit of reactive powers of compensation devices inthe i^(th) wind farm;

The second determining unit is configured to determine second boundaryconditions (7) for the third objective function:

$\begin{matrix}{{{{s.t.\mspace{14mu} \underset{\_}{V_{{PCC},i}}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{n \in w_{i}}\left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{i,n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{i,n}}\Delta \; Q_{n}}} \right)}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w}}{{\underset{\_}{V_{{PCC},i}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{{n \in w},{n \neq s}}\left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}\Delta \; Q_{n}}} \right)} + {\frac{\partial V_{{PCC},i}^{0}}{\partial P_{s}}\Delta \; P_{s}^{s}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{s}}\Delta \; Q_{s}^{s}}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w},{i \neq s},{\in S}}{{\underset{\_}{\Delta \; P_{i}} \leq {\Delta \; P_{i}} \leq \overset{\_}{\Delta \; P_{i}}},{i \in w}}{{\underset{\_}{\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\min}} \leq {\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\max}} \leq \overset{\_}{\Delta \; Q_{wi}}},{i \in w}}{{\underset{\_}{\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\min}} \leq {\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\max}} \leq \overset{\_}{\Delta \; Q_{ci}}},{i \in w}}} & (7)\end{matrix}$

where

$\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}$

represent a voltage sensitivity coefficient of an active power of an^(th) wind farm relative to the output bus of the i^(th) wind farm anda voltage sensitivity coefficient of a reactive power of the n^(th) windfarm relative to the output bus of the i^(th) wind farm respectively,ΔP_(n) and ΔQ_(n) represent a variation of the active power of then^(th) wind farm and a variation of the reactive power of the n^(th)wind farm respectively, n≠i, w is a set of wind farms in the wind farmconverging area, S is a set of tripped off wind farms, ΔP_(s) ^(s) andΔQ_(s) ^(s) represent a variation of active powers of tripped off windfarms and a variation of reactive powers of tripped off wind farmsrespectively;

The second resolving unit is configured to resolve the third objectivefunction based on the second boundary conditions to obtain the secondlower limit and the second upper limit of the reactive powers of thefans in each wind farm, the second lower limit and the second upperlimit of the reactive powers of the compensation devices in each windfarm.

In an embodiment, the fourth obtaining module is further configured toobtain the voltage error according to formula (8):

$\begin{matrix}{{\Delta \; V_{S}} = {{\sum\limits_{i \in w}\; {{\overset{\_}{V_{{PCC},i}^{({m + 1})}} - \overset{\_}{V_{{PCC},i}^{(m)}}}}} + {\sum\limits_{i \in w}\; {{\underset{\_}{V_{{PCC},i}^{({m + 1})}} - \underset{\_}{V_{{PCC},i}^{(m)}}}}}}} & (8)\end{matrix}$

where V_(PCC,i) ^((m)) is an upper limit voltage of the output bus ofthe i^(th) wind farm in a m^(th) time iteration and V_(PCC,i) ^((m)) isa lower limit voltage of the output bus of the i^(th) wind farm in them^(th) time iteration.

The present disclosure provides a computer readable storage medium,comprising a computer program for executing a method for determining asafe threshold for power grid voltage of a wind farm converging area, inwhich the method includes:

step A: obtaining a ground state voltage of an output bus of each windfarm in the wind farm converging area;

step B: obtaining an upper limit voltage and a lower limit voltage ofthe output bus of each wind farm, a first lower limit and a first upperlimit of reactive powers of fans in each wind farm, a first lower limitand a first upper limit of reactive powers of compensation devices ineach wind farm, and a lower limit and an upper limit of active powerfluctuations of the fans in each wind farm according to the ground statevoltage of the output bus of each wind farm;

step C: obtaining a second lower limit and a second upper limit of thereactive powers of the fans in each wind farm, a second lower limit anda second upper limit of the reactive powers of the compensation devicesin each wind farm according to the upper limit voltage and the lowerlimit voltage of the output bus of each wind farm, the first lower limitand the first upper limit of the reactive powers of the fans in eachwind farm, the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm, and thelower limit and the upper limit of the active power fluctuations of thefans in each wind farm;

step D: obtaining a voltage error according to the upper limit voltageand the lower limit voltage of the output bus of each wind farm;

step E: determining whether the voltage error is greater than apredetermined threshold;

step F: replacing the first lower limit and the first upper limit of thereactive powers of the fans in each wind farm, and the first lower limitand the first upper limit of the reactive powers of the compensationdevices in each wind farm with the second lower limit and the secondupper limit of the reactive powers of the fans in each wind farm, thesecond lower limit and the second upper limit of the reactive powers ofthe compensation devices in each wind farm respectively, and repeatingsteps B-F to perform a time iteration, if the voltage error is greaterthan the predetermined threshold;

step G: defining the upper limit voltage and the lower limit voltage ofthe output bus of each wind farm as a safe threshold of each wind farm,if the voltage error is less than or equal to the predeterminedthreshold.

Although explanatory embodiments have been shown and described, it wouldbe appreciated by those skilled in the art that the above embodimentscannot be construed to limit the present disclosure, and changes,alternatives, and modifications can be made in the embodiments withoutdeparting from spirit, principles and scope of the present disclosure.

What is claimed is:
 1. A method for determining a safe threshold forpower grid voltage of a wind farm converging area, comprising thefollowing acts performed by an automatic voltage control system of thepower grid: step A: obtaining a ground state voltage of an output bus ofeach wind farm in the wind farm converging area; step B: obtaining anupper limit voltage and a lower limit voltage of the output bus of eachwind farm, a first lower limit and a first upper limit of reactivepowers of fans in each wind farm, a first lower limit and a first upperlimit of reactive powers of compensation devices in each wind farm, anda lower limit and an upper limit of active power fluctuations of thefans in each wind farm according to the ground state voltage of theoutput bus of each wind farm; step C: obtaining a second lower limit anda second upper limit of the reactive powers of the fans in each windfarm, a second lower limit and a second upper limit of the reactivepowers of the compensation devices in each wind farm according to theupper limit voltage and the lower limit voltage of the output bus ofeach wind farm, the first lower limit and the first upper limit of thereactive powers of the fans in each wind farm, the first lower limit andthe first upper limit of the reactive powers of the compensation devicesin each wind farm, and the lower limit and the upper limit of the activepower fluctuations of the fans in each wind farm; step D: obtaining avoltage error according to the upper limit voltage and the lower limitvoltage of the output bus of each wind farm; step E: determining whetherthe voltage error is greater than a predetermined threshold; step F:replacing the first lower limit and the first upper limit of thereactive powers of the fans in each wind farm, and the first lower limitand the first upper limit of the reactive powers of the compensationdevices in each wind farm with the second lower limit and the secondupper limit of the reactive powers of the fans in each wind farm, thesecond lower limit and the second upper limit of the reactive powers ofthe compensation devices in each wind farm respectively, and repeatingsteps B-F to perform a time iteration, if the voltage error is greaterthan the predetermined threshold; step G: defining the upper limitvoltage and the lower limit voltage of the output bus of each wind farmas a safe threshold of each wind farm, if the voltage error is less thanor equal to the predetermined threshold.
 2. The method according toclaim 1, wherein step A comprises: setting each reactive power of eachfan in each wind farm to be 0, and setting each reactive power of eachcompensation device in each wind farm to be 0; obtaining the groundstate voltage of the output bus of each wind farm by performing a groundstate flow power calculation.
 3. The method according to claim 1,wherein step B comprises: establishing a first objective function (1) ofthe upper limit voltage of the output bus of each wind farm and a secondobjective function (2) of the lower limit voltage of the output bus ofeach wind farm: $\begin{matrix}{\overset{\_}{V_{{PCC},i}} = {{\underset{\Delta \; p_{w_{i}}}{\min \;}{\max\limits_{{\Delta q}_{w_{i}},{\Delta \; q_{c_{i}}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{j \in c_{i}}{\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (1) \\{\underset{\_}{V_{{PCC},i}} = {{\underset{\Delta \; p_{w_{i}}}{\max \;}{\min\limits_{{\Delta q}_{w_{i}},{\Delta \; q_{c_{i}}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}{\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (2)\end{matrix}$ where V_(PCC,i) is an upper limit voltage of the outputbus of a i^(th) wind farm, V_(PCC,i) is a lower limit voltage of theoutput bus of the i^(th) wind farm, V_(PCC,i) ⁰ is a ground state of thei^(th) wind farm, w_(i) is a set of fans in the i^(th) wind farm, c_(i)is a set of compensation devices in the i^(th) wind farm,$\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}$represent a voltage sensitivity coefficient of an active power of aj^(th) fan in the i^(th) wind farm relative to V_(PCC,i) ⁰ and a voltagesensitivity coefficient of a reactive power of the j^(th) fan in thei^(th) wind farm relative to V_(PCC,i) ⁰ respectively,$\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}$ represents a voltagesensitivity coefficient of a reactive power of a k^(th) compensationdevice in the i^(th) wind farm relative to V_(PCC,i) ⁰, Δp_(i,j) andΔq_(i,j) represent a variation of the active power of the j^(th) fan inthe i^(th) wind farm and a variation of the reactive power of the j^(th)fan in the i^(th) wind farm respectively, Δq_(i,k) represents avariation of the reactive power of the k^(th) compensation device in thei^(th) wind farm; determining first boundary conditions (3) for thefirst objective function and the second objective function:$\begin{matrix}{\mspace{734mu} {(3){{{s.t.\mspace{14mu} \underset{\_}{V_{i,t}^{0}}} \leq {V_{i,t}^{0} + {\sum\limits_{j \in w_{i}}\left( {{\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}{\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}} \leq \overset{\_}{V_{i,t}^{0}}},{{t \in w_{i}}{{\underset{\_}{\Delta \; p_{i,t}} \leq {\Delta \; p_{i,t}} \leq \overset{\_}{\Delta \; p_{i,t}}},{t \in w_{i}}}{{\underset{\_}{\Delta \; q_{i,t}} \leq {\Delta \; q_{i,t}} \leq \overset{\_}{\Delta \; q_{i,t}}},{t \in w_{i}}}{{\underset{\_}{\Delta \; q_{i,k}} \leq {\Delta \; q_{i,k}} \leq \overset{\_}{\Delta \; q_{i,k}}},{k \in c_{i}}}{\underset{\_}{\Delta \; Q_{wi}} \leq \underset{t \in w_{i}}{\sum{\Delta \; q_{i,t}}} \leq \overset{\_}{\Delta \; Q_{wi}}}{\underset{\_}{\Delta \; Q_{ci}} \leq \underset{k \in c_{i}}{\sum{\Delta \; q_{i,k}}} \leq \overset{\_}{\Delta \; Q_{ci}}}}}}} & \;\end{matrix}$ where V_(i,t) ⁰ and V_(i,t) ⁰ represent a lower limitvoltage and upper limit voltage in a normal operation of a t^(th) fan inthe i^(th) wind farm respectively, t≠j,$\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}$represent a voltage sensitivity coefficient of the active power of thej^(th) fan in the i^(th) wind farm relative to the t^(th) fan and avoltage sensitivity coefficient of the reactive power of the j^(th) fanin the i^(th) wind farm relative to the t^(th) fan respectively,$\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}$ represents a voltagesensitivity coefficient of the reactive power of the k^(th) compensationdevice in the i^(th) wind farm relative to the t^(th) fan, Δp_(i,t) andΔp_(i,t) represent a lower limit and an upper limit of an active powerfluctuation of the t^(th) fan in the i^(th) wind farm respectively,Δq_(i,t) and Δq_(i,t) represent a lower limit and an upper limit of areactive power fluctuation of the t^(th) fan in the i^(th) wind farmrespectively, Δq_(i,k) and Δq_(i,k) represent a lower limit and an upperlimit of a reactive power fluctuation of the k^(th) compensation devicein the i^(th) wind farm respectively, ΔQ_(wi) and ΔQ_(wi) represent alower limit and an upper limit of reactive powers of fans in the i^(th)wind farm, ΔQ_(ci) and ΔQ_(ci) represent a lower limit and an upperlimit of reactive powers of compensation devices in the i^(th) windfarm; calculating a lower limit of the active power fluctuation of thei^(th) wind farm according to formula (4): $\begin{matrix}{{\underset{\_}{\Delta \; P_{i}} = {\sum\limits_{t \in w_{i}}\; \underset{\_}{\Delta \; p_{i,t}}}},} & (4)\end{matrix}$ and calculating an upper limit of the active powerfluctuation of the i^(th) wind farm according to formula (5):$\begin{matrix}{{\overset{\_}{\Delta \; P_{i}} = {\sum\limits_{t \in w_{i}}\; \overset{\_}{\Delta \; p_{i,t}}}},} & (5)\end{matrix}$ where ΔP_(i) and ΔP_(i) represent the lower limit and theupper limit of the active power fluctuation of the i^(th) wind farmrespectively; resolving the first objective function and the secondobjective function based on the first boundary conditions to obtain theupper limit voltage and the lower limit voltage of the output bus ofeach wind farm, the first lower limit and the first upper limit of thereactive powers of the fans in each wind farm, the first lower limit andthe first upper limit of the reactive powers of the compensation devicesin each wind farm, and the lower limit and the upper limit of the activepower fluctuations of the fans in each wind farm.
 4. The methodaccording to claim 3, wherein step C comprises: establishing a thirdobjective function (6) of reactive power in the wind farm convergingarea according to the upper limit voltage and the lower limit voltage ofthe output bus of each wind farm, the first lower limit and the firstupper limit of the reactive powers of the fans in each wind farm, thefirst lower limit and the first upper limit of the reactive powers ofthe compensation devices in each wind farm, and the lower limit and theupper limit of the active power fluctuation of the fans in each windfarm: $\begin{matrix}{{\Delta \; Q} = {{\max\limits_{\Delta \; P}\mspace{11mu} {\min\limits_{{\Delta \; Q_{wi}},{\Delta \; Q_{ci}}}{\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{wi}^{\max}} - {\Delta \; Q_{wi}^{\min}}} \right)}}} + {\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{ci}^{\max}} - {\Delta \; Q_{ci}^{\min}}} \right)}}} & (6)\end{matrix}$ where ΔQ_(wi) ^(max) and ΔQ_(wi) ^(min) represent a secondlower limit and a second upper limit of reactive powers of fans in thei^(th) wind farm, ΔQ_(ci) ^(max) and ΔQ_(ci) ^(min) represent a secondlower limit and a second upper limit of reactive powers of compensationdevices in the i^(th) wind farm; determining second boundary conditions(7) for the third objective function: $\begin{matrix}{{{{s.t.\mspace{14mu} \underset{\_}{V_{{PCC},i}}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{n \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{i,n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{i,n}}\Delta \; Q_{n}}} \right)}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w}}{{\underset{\_}{V_{{PCC},i}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{{n \in w},{n \neq s}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}\Delta \; Q_{n}}} \right)} + {\frac{\partial V_{{PCC},i}^{0}}{\partial P_{s}}\Delta \; P_{s}^{s}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{s}}\Delta \; Q_{s}^{s}}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w},{i \neq s},{\in S}}{{\underset{\_}{\Delta \; P_{i}} \leq {\Delta \; P_{i}} \leq \overset{\_}{\Delta \; P_{i}}},\mspace{31mu} {i \in w}}{{\underset{\_}{\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\min}} \leq {\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\max}} \leq \overset{\_}{\Delta \; Q_{wi}}},\mspace{14mu} {i \in w}}{{\underset{\_}{\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\min}} \leq {\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\max}} \leq \overset{\_}{\Delta \; Q_{ci}}},\mspace{14mu} {i \in w}}} & (7)\end{matrix}$ where$\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}$represent a voltage sensitivity coefficient of an active power of an^(th) wind farm relative to the output bus of the i^(th) wind farm anda voltage sensitivity coefficient of a reactive power of the n^(th) windfarm relative to the output bus of the i^(th) wind farm respectively,ΔP_(n) and ΔQ_(n) represent a variation of the active power of then^(th) wind farm and a variation of the reactive power of the n^(th)wind farm respectively, n≠i, w is a set of wind farms in the wind farmconverging area, S is a set of tripped off wind farms, ΔP_(s) ^(s) andΔQ_(s) ^(s) represent a variation of active powers of tripped off windfarms and a variation of reactive powers of tripped off wind farmsrespectively; resolving the third objective function based on the secondboundary conditions to obtain the second lower limit and the secondupper limit of the reactive powers of the fans in each wind farm, thesecond lower limit and the second upper limit of the reactive powers ofthe compensation devices in each wind farm.
 5. The method according toclaim 4, wherein obtaining the voltage error according to formula (8):$\begin{matrix}{{\Delta \; V_{S}} = {{\sum\limits_{i \in w}\; {{\overset{\_}{V_{{PCC},i}^{({m + 1})}} - \overset{\_}{V_{{PCC},i}^{(m)}}}}} + {\sum\limits_{i \in w}\; {{\underset{\_}{V_{{PCC},i}^{({m + 1})}} - \underset{\_}{V_{{PCC},i}^{(m)}}}}}}} & (8)\end{matrix}$ where V_(PCC,i) ^((m)) is an upper limit voltage of theoutput bus of the i^(th) wind farm in a m^(th) time iteration andV_(PCC,i) ^((m)) is a lower limit voltage of the output bus of thei^(th) wind farm in the m^(th) time iteration.
 6. A device fordetermining a safe threshold for power grid voltage of a wind farmconverging area, comprising: a non-transitory computer-readable mediumcomprising computer-executable instructions stored thereon; and aninstruction execution system, which is configured by the instructions toimplement at least one of following modules: a first obtaining module,configured to obtain a ground state voltage of an output bus of eachwind farm in the wind farm converging area; a second obtaining module,configured to obtain an upper limit voltage and a lower limit voltage ofthe output bus of each wind farm, a first lower limit and a first upperlimit of reactive powers of fans in each wind farm, a first lower limitand a first upper limit of reactive powers of compensation devices ineach wind farm, and a lower limit and an upper limit of active powerfluctuations of the fans in each wind farm according to the ground statevoltage of the output bus of each wind farm; a third obtaining module,configured to obtain a second lower limit and a second upper limit ofthe reactive powers of the fans in each wind farm, a second lower limitand a second upper limit of the reactive powers of the compensationdevices in each wind farm according to the upper limit voltage and thelower limit voltage of the output bus of each wind farm, the first lowerlimit and the first upper limit of the reactive powers of the fans ineach wind farm, the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm, and thelower limit and the upper limit of the active power fluctuations of thefans in each wind farm; a fourth obtaining module, configured to obtaina voltage error according to the upper limit voltage and the lower limitvoltage of the output bus of each wind farm; a determining module,configured to determine whether the voltage error is greater than apredetermined threshold; an iteration module, configured to replace thefirst lower limit and the first upper limit of the reactive powers ofthe fans in each wind farm, and the first lower limit and the firstupper limit of the reactive powers of the compensation devices in eachwind farm with the second lower limit and the second upper limit of thereactive powers of the fans in each wind farm, the second lower limitand the second upper limit of the reactive powers of the compensationdevices in each wind farm respectively, and to repeat to perform a timeiteration, if the voltage error is greater than the predeterminedthreshold; a defining module, configured to define the upper limitvoltage and the lower limit voltage of the output bus of each wind farmas a safe threshold of each wind farm, if the voltage error is less thanor equal to the predetermined threshold.
 7. The device according toclaim 6, wherein the first obtaining module comprises: a setting unit,configured to set each reactive power of each fan in each wind farm tobe 0, and to set each reactive power of each compensation device in eachwind farm to be 0; a first obtaining unit, configured to obtain theground state voltage of the output bus of each wind farm by performing aground state flow power calculation.
 8. The device according to claim 6,wherein the second obtaining module comprises: a first establishingunit, configured to establish a first objective function (1) of theupper limit voltage of the output bus of each wind farm and a secondobjective function (2) of the lower limit voltage of the output bus ofeach wind farm: $\begin{matrix}{\overset{\_}{V_{{PCC},i}} = {{\min\limits_{\Delta \; p_{w_{i}}}{\max\limits_{{\Delta \; q_{w_{i}}},{\Delta \; q_{c_{i}}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{j \in c_{i}}\; {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (1) \\{\overset{\_}{V_{{PCC},i}} = {{\max\limits_{\Delta \; p_{w_{i}}}{\min\limits_{{\Delta \; q_{w_{i}}},{\Delta \; q_{c_{i}}}}V_{{PCC},i}^{0}}} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}\; {\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}}} & (2)\end{matrix}$ where V_(PCC,i) is an upper limit voltage of the outputbus of a i^(th) wind farm, V_(PCC,i) is a lower limit voltage of theoutput bus of the i^(th) wind farm, V_(PCC,i) ⁰ is a ground state of thei^(th) wind farm, w_(i) is a set of fans in the i^(th) wind farm, c_(i)is a set of compensation devices in the i^(th) wind farm,$\frac{\partial V_{{PCC},i}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,j}}$represent a voltage sensitivity coefficient of an active power of aj^(th) fan in the i^(th) wind farm relative to V_(PCC,i) ⁰ and a voltagesensitivity coefficient of a reactive power of the j^(th) fan in thei^(th) wind farm relative to V_(PCC,i) ⁰ respectively,$\frac{\partial V_{{PCC},i}^{0}}{\partial q_{i,k}}$ represents a voltagesensitivity coefficient of a reactive power of a k^(th) compensationdevice in the i^(th) wind farm relative to V_(PCC,i) ⁰, Δp_(i,j) andΔq_(i,j) represent a variation of the active power of the j^(th) fan inthe i^(th) wind farm and a variation of the reactive power of the j^(th)fan in the i^(th) wind farm respectively, Δq_(i,k) represents avariation of the reactive power of the k^(th) compensation device in thei^(th) wind farm; a first determining unit, configured to determinefirst boundary conditions (3) for the first objective function and thesecond objective function: $\begin{matrix}{{{{s.t.\mspace{14mu} \underset{\_}{V_{i,t}^{0}}} \leq {V_{i,t}^{0} + {\sum\limits_{j \in w_{i}}\; \left( {{\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\Delta \; p_{i,j}} + {\frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}\Delta \; q_{i,j}}} \right)} + {\sum\limits_{k \in c_{i}}\; {\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}\Delta \; q_{i,k}}}} \leq \overset{\_}{V_{i,t}^{0}}},{t \in w_{i}}}{{\underset{\_}{\Delta \; p_{i,t}} \leq {\Delta \; p_{i,t}} \leq \overset{\_}{\Delta \; p_{i,t}}},\mspace{14mu} {t \in w_{i}}}{{\underset{\_}{\Delta \; q_{i,t}} \leq {\Delta \; q_{i,t}} \leq \overset{\_}{\Delta \; q_{i,t}}},\mspace{14mu} {t \in w_{i}}}{{\underset{\_}{\Delta \; q_{i,k}} \leq {\Delta \; q_{i,k}} \leq \overset{\_}{\Delta \; q_{i,k}}},\mspace{14mu} {k \in c_{i}}}{\underset{\_}{\Delta \; Q_{wi}} \leq {\sum\limits_{t \in w_{i}}\; {\Delta \; q_{i,t}}} \leq \overset{\_}{\Delta \; Q_{w,i}}}{\underset{\_}{\Delta \; Q_{ci}} \leq {\sum\limits_{k \in c_{i}}\; {\Delta \; q_{i,k}}} \leq \overset{\_}{\Delta \; Q_{c,i}}}} & (3)\end{matrix}$ where V_(i,t) ⁰ and V_(i,t) ⁰ represent a lower limitvoltage and upper limit voltage in a normal operation of a t^(th) fan inthe i^(th) wind farm respectively, t≠j,$\frac{\partial V_{i,t}^{0}}{\partial p_{i,j}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{i,t}^{0}}{\partial q_{i,j}}$represent a voltage sensitivity coefficient of the active power of thej^(th) fan in the i^(th) wind farm relative to the t^(th) fan and avoltage sensitivity coefficient of the reactive power of the j^(th) fanin the i^(th) wind farm relative to the t^(th) fan respectively,$\frac{\partial V_{i,t}^{0}}{\partial q_{i,k}}$ represents a voltagesensitivity coefficient of the reactive power of the k^(th) compensationdevice in the i^(th) wind farm relative to the t^(th) fan, Δp_(i,t) andΔp_(i,t) represent a lower limit and an upper limit of an active powerfluctuation of the t^(th) fan in the i^(th) wind farm respectively,Δq_(i,t) and Δq_(i,t) represent a lower limit and an upper limit of areactive power fluctuation of the t^(th) fan in the i^(th) wind farmrespectively, Δq_(i,k) and Δq_(i,k) represent a lower limit and an upperlimit of a reactive power fluctuation of the k^(th) compensation devicein the i^(th) wind farm respectively, ΔQ_(wi) and ΔQ_(wi) represent alower limit and an upper limit of reactive powers of fans in the i^(th)wind farm, ΔQ_(ci) and ΔQ_(ci) represent a lower limit and an upperlimit of reactive powers of compensation devices in the i^(th) windfarm; a first calculating unit, configured to calculate a lower limit ofthe active power fluctuation of the i^(th) wind farm according toformula (4): $\begin{matrix}{{\underset{\_}{\Delta \; P_{i}} \leq {\sum\limits_{t \in w_{i}}\; \underset{\_}{\Delta \; p_{i,t}}}},} & (4)\end{matrix}$ and to calculate an upper limit of the active powerfluctuation of the i^(th) wind farm according to formula (5):$\begin{matrix}{{\underset{\_}{\Delta \; P_{i}} \leq {\sum\limits_{t \in w_{i}}\; \overset{\_}{\Delta \; p_{i,t}}}},} & (5)\end{matrix}$ where ΔP_(i) and ΔP_(i) represent the lower limit and theupper limit of the active power fluctuation of the i^(th) wind farmrespectively; a first resolving unit, configured to resolve the firstobjective function and the second objective function based on the firstboundary conditions to obtain the upper limit voltage and the lowerlimit voltage of the output bus of each wind farm, the first lower limitand the first upper limit of the reactive powers of the fans in eachwind farm, the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm, and thelower limit and the upper limit of the active power fluctuations of thefans in each wind farm.
 9. The device according to claim 8, wherein thethird obtaining module comprises: a second establishing unit, configuredto establish a third objective function (6) of reactive power in thewind farm converging area according to the upper limit voltage and thelower limit voltage of the output bus of each wind farm, the first lowerlimit and the first upper limit of the reactive powers of the fans ineach wind farm, the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm, and thelower limit and the upper limit of the active power fluctuation of thefans in each wind farm: $\begin{matrix}{{\Delta \; Q} = {{\max\limits_{\Delta \; P}{\min\limits_{{\Delta \; Q_{wi}},{\Delta \; Q_{ci}}}{\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{wi}^{\max}} - {\Delta \; Q_{wi}^{\min}}} \right)}}} + {\sum\limits_{i \in w}\; \left( {{\Delta \; Q_{ci}^{\max}} - {\Delta \; Q_{ci}^{\min}}} \right)}}} & (6)\end{matrix}$ where ΔQ_(wi) ^(max) and ΔQ_(wi) ^(min) represent a secondlower limit and a second upper limit of reactive powers of fans in thei^(th) wind farm, ΔQ_(ci) ^(max) and ΔQ_(ci) ^(min) represent a secondlower limit and a second upper limit of reactive powers of compensationdevices in the i^(th) wind farm; a second determining unit, configuredto determine second boundary conditions (7) for the third objectivefunction: $\begin{matrix}{{{{s.t.\mspace{14mu} \underset{\_}{V_{{PCC},i}}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{n \in w_{i}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{i,n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{i,n}}\Delta \; Q_{n}}} \right)}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w}}{{\underset{\_}{V_{{PCC},i}} \leq {V_{{PCC},i}^{0} + {\sum\limits_{{n \in w},{n \neq s}}\; \left( {{\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\Delta \; P_{n}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}\Delta \; Q_{n}}} \right)} + {\frac{\partial V_{{PCC},i}^{0}}{\partial P_{s}}\Delta \; P_{s}^{s}} + {\frac{\partial V_{{PCC},i}^{0}}{\partial Q_{s}}\Delta \; Q_{s}^{s}}} \leq \overset{\_}{V_{{PCC},i}}},{i \in w},{i \neq s},{\in S}}\; {{\underset{\_}{\Delta \; P_{i}} \leq {\Delta \; P_{i}} \leq \overset{\_}{\Delta \; P_{i}}},\mspace{31mu} {i \in w}}{{\underset{\_}{\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\min}} \leq {\Delta \; Q_{wi}} \leq {\Delta \; Q_{wi}^{\max}} \leq \overset{\_}{\Delta \; Q_{wi}}},\mspace{14mu} {i \in w}}{{\underset{\_}{\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\min}} \leq {\Delta \; Q_{ci}} \leq {\Delta \; Q_{ci}^{\max}} \leq \overset{\_}{\Delta \; Q_{ci}}},\mspace{14mu} {i \in w}}} & (7)\end{matrix}$ where$\frac{\partial V_{{PCC},i}^{0}}{\partial P_{n}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial V_{{PCC},i}^{0}}{\partial Q_{n}}$represent a voltage sensitivity coefficient of an active power of an^(th) wind farm relative to the output bus of the i^(th) wind farm anda voltage sensitivity coefficient of a reactive power of the n^(th) windfarm relative to the output bus of the i^(th) wind farm respectively,ΔP_(n) and ΔQ_(n) represent a variation of the active power of then^(th) wind farm and a variation of the reactive power of the n^(th)wind farm respectively, n≠i, w is a set of wind farms in the wind farmconverging area, S is a set of tripped off wind farms, ΔP_(s) ^(s) andΔQ_(s) ^(s) represent a variation of active powers of tripped off windfarms and a variation of reactive powers of tripped off wind farmsrespectively; a second resolving unit, configured to resolve the thirdobjective function based on the second boundary conditions to obtain thesecond lower limit and the second upper limit of the reactive powers ofthe fans in each wind farm, the second lower limit and the second upperlimit of the reactive powers of the compensation devices in each windfarm.
 10. The device according to claim 9, wherein the fourth obtainingmodule is further configured to obtain the voltage error according toformula (8): $\begin{matrix}{{\Delta \; V_{S}} = {{\sum\limits_{i \in w}\; {{\overset{\_}{V_{{PCC},i}^{({m + 1})}} - \overset{\_}{V_{{PCC},i}^{(m)}}}}} + {\sum\limits_{i \in w}\; {{\underset{\_}{V_{{PCC},i}^{({m + 1})}} - \underset{\_}{V_{{PCC},i}^{(m)}}}}}}} & (8)\end{matrix}$ where V_(PCC,i) ^((m)) is an upper limit voltage of theoutput bus of the i^(th) wind farm in a m^(th) time iteration andV_(PCC,i) ^((m)) is a lower limit voltage of the output bus of thei^(th) wind farm in the m^(th) time iteration.
 11. A non-transitorycomputer-readable storage medium having stored therein instructionsthat, when executed by a processor of a computer, causes the computer toperform a method for determining a safe threshold for power grid voltageof a wind farm converging area, the method comprising: step A: obtaininga ground state voltage of an output bus of each wind farm in the windfarm converging area; step B: obtaining an upper limit voltage and alower limit voltage of the output bus of each wind farm, a first lowerlimit and a first upper limit of reactive powers of fans in each windfarm, a first lower limit and a first upper limit of reactive powers ofcompensation devices in each wind farm, and a lower limit and an upperlimit of active power fluctuations of the fans in each wind farmaccording to the ground state voltage of the output bus of each windfarm; step C: obtaining a second lower limit and a second upper limit ofthe reactive powers of the fans in each wind farm, a second lower limitand a second upper limit of the reactive powers of the compensationdevices in each wind farm according to the upper limit voltage and thelower limit voltage of the output bus of each wind farm, the first lowerlimit and the first upper limit of the reactive powers of the fans ineach wind farm, the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm, and thelower limit and the upper limit of the active power fluctuations of thefans in each wind farm; step D: obtaining a voltage error according tothe upper limit voltage and the lower limit voltage of the output bus ofeach wind farm; step E: determining whether the voltage error is greaterthan a predetermined threshold; step F: replacing the first lower limitand the first upper limit of the reactive powers of the fans in eachwind farm, and the first lower limit and the first upper limit of thereactive powers of the compensation devices in each wind farm with thesecond lower limit and the second upper limit of the reactive powers ofthe fans in each wind farm, the second lower limit and the second upperlimit of the reactive powers of the compensation devices in each windfarm respectively, and repeating steps B-F to perform a time iteration,if the voltage error is greater than the predetermined threshold; stepG: defining the upper limit voltage and the lower limit voltage of theoutput bus of each wind farm as a safe threshold of each wind farm, ifthe voltage error is less than or equal to the predetermined threshold.